Transformers are necessary in many types of electrical apparatus.
A basic transformer comprises primary coils, secondary coils and a core. The coils are made, for example, of copper wire. They may be arranged so as to be juxtaposed. Alternatively, they may be arranged so that one coil surrounds another coil. A coil has one or more windings. As a result of the ongoing reduction in size of electrical apparatus, transformers manufactured also comprise coils having smaller dimensions. Said coils may be made, for example, from a number of layers of an insulating material on which winding portions of the coils are provided. A transformer of this type is referred to as a multilayer or planar transformer.
FIG. 1A is the top view of a conventional planar transformer. FIG. 1B shows an exploded top view of the conventional planar transformer. In this type of transformer, the primary windings 100, which are a spiral of traces on a planar surface 104, are coupled to the secondary windings 102, which are a different spiral of traces on a separate planar surface 106. A thin dielectric insulator 112 is located between the first windings 100 and secondary windings 102. Magnetic cores 108 and 110 enclose the windings 100 and 102 and the insulator 112. Typically, the magnetic core is made of ferrite or some other composite material that is shaped as a pot-core, an R-M core, an E core, an I core, etc. However, the core can be almost any shape that is easy to place around the windings and effectively confines the magnetic field to the area around the windings.
FIG. 1C shows an exploded bottom view of the conventional planar transformer. The first windings 100 through the hole 114 provide an input end 118 under the planar surface 104. The second windings 102 through the hole 116 provide an output end 120 under the planar surface 106. If a current is sent to the input end 118 of the first windings 100, magnetic flux causes a current to be generated in the secondary windings 102. The core conducts this flux since it is made of a material having good magneto-conductive properties. During operation, the input end 118 is connected to the power supply and the output end is connected to the current circuit of the apparatus receiving energy from the power supply.
The use of planar traces rather than the classical wire windings on a bobbin is a significant manufacturing advance for high-frequency transformers. However, in accordance with the conventional structure, a two-layer printed circuit board is required to build each winding. In other words, realization of a planar transformer with N windings requires 2N-layers printed circuit board. This increases the manufacture cost. As a result of the ongoing reduction in cost of equipment, a further reduction in cost of the planar transformers is desirable.